nir/lower_goto_ifs: Don't destroy SSA form in the process

There are two issues here:

 1. If there are any phi nodes, we'll make complete hash of them.  This
    isn't likely actually a problem because spirv_to_nir doesn't
    generate any actual phi nodes today.  However, if we start doing any
    other passes before this, we may have a problem.

 2. Even without phi nodes, we may still break SSA form.  This can
    happen if we ever have to stick a block inside a conditional to
    satisfy weird CFG constraints.  Doing so can cause it to no longer
    look like it dominates some of its uses even though, at runtime,
    it's guaranteed to be run first.

Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6750>

src/compiler/nir/nir_lower_goto_ifs.c

@@ -957,6 +957,12 @@ nir_lower_goto_ifs_impl(nir_function_impl *impl)
 
    nir_metadata_require(impl, nir_metadata_dominance);
 
+   /* We're going to re-arrange blocks like crazy.  This is much easier to do
+    * if we don't have any phi nodes to fix up.
+    */
+   nir_foreach_block_unstructured(block, impl)
+      nir_lower_phis_to_regs_block(block);
+
    nir_cf_list cf_list;
    nir_cf_extract(&cf_list, nir_before_cf_list(&impl->body),
                             nir_after_cf_list(&impl->body));
@@ -997,6 +1003,9 @@ nir_lower_goto_ifs_impl(nir_function_impl *impl)
 
    nir_metadata_preserve(impl, nir_metadata_none);
 
+   nir_repair_ssa_impl(impl);
+   nir_lower_regs_to_ssa_impl(impl);
+
    return true;
 }